This invention relates in general to measurement apparatus; more particularly, to a time measurement system for analyzing components of a signal.
A measurement apparatus is disclosed in U.S. Pat. No. 4,908,784, which is hereby incorporated by reference. A typical measurement apparatus is the Wavecrest DTS-2075, available from Wavecrest Corporation, Edina, Minn. A measurement apparatus measures the time interval between two edge transitions through counters. Known measurement apparatus such as the Wavecrest DTS-2075 have two dedicated synchronous counters that take in every edge of signal under test in order to identify the edges to be measured. For each run of the measurement apparatus, only a xe2x80x9cstartxe2x80x9d edge and a xe2x80x9cstopxe2x80x9d edge may be measured. Identifying and accurately measuring the time of a particular edge becomes more difficult as input frequencies increase.
One solution, used in the DTS-2075, is to re-clock the terminal count output of the synchronous counters with a delayed version of the input signal. This method requires that the counters run at the input frequency and that the wideband input signal be delayed without significant distortion. These two requirements become difficult to meet for high frequency inputs. If this method is extended to allow the measurement of many edges, it requires the high speed signal to be split to many counters, which results in either lower signal amplitudes which result in higher measurement jitter, or the use of wideband amplifier stages that are difficult to implement without either great expense or some signal distortion which results in time measurement errors.
There is a need for a simplified measurement apparatus that is capable of high frequency operation with low error. There is a also a need for a measurement apparatus that is capable of measuring many edges of a high frequency signal during one test run.
In accordance with the present invention, the above and other problems are solved by a system that allows time measurement of the Nth edge after arming while minimizing the signal integrity problems associated with propagating high-frequency signals. The system includes a frequency divider circuit that can be cleared by an external signal, a multiplexer coupled to the divider circuit driven by an output edge its inverse, and a counter circuit coupled to the multiplexer driven by outputs of the multiplexer. A time quantizer may be coupled to the counter outputs.
The system according to this invention allows a higher frequency of operation by reducing the output frequency of the parts. It reduces the number of high-frequency components, which may allow a cost savings by allowing the use of lower-speed parts and which may allow the circuit to be integrated more cost-effectively. It generates multiple outputs to allow time measurements of multiple edges in a data stream or clock signal. It has multiple time stamping modules (time quantizers) that can be assigned to measure the times of arbitrary edges on any input.
In one embodiment, ripple counters rather than synchronous counters implement the xe2x80x98Arm on Nth eventxe2x80x99 counters. Higher input frequencies are allowed by using the counter stages as frequency dividers, thereby avoiding the need to propagate highfrequency signals through the arming circuit and providing a scalable method for increasing operating frequencies in the future. The ripple counter will essentially xe2x80x98swallowxe2x80x99 the first N-1 edges after the arming signal, and output its terminal count signal on the Nth edge. The synchronous terminal count signal is then used to trigger the ramps.
In another embodiment, the high-frequency input is split into parallel, lower frequency signals using a cascade of frequency dividers. Choosing the appropriate lower frequency signal path corresponds to choosing a certain preset value of a ripple counter. This embodiment allows the frequency-divided signals to be split to drive many timers, avoiding many of the signal integrity problems associated with splitting the high-frequency input signal many ways.
In accordance with other aspects, the present invention relates to a time measurement apparatus including an event counter circuit and time quantizers. The event counter circuit includes a frequency divider circuit that can be cleared by an external signal, a multiplexer coupled to the divider circuit driven by an output edge and its inverse, and a counter circuit coupled to the multiplexer driven by outputs of the multiplexer.
The great utility of the invention is that measurement apparatus provides for increased measurement rates and input frequencies and measurements of multiple edges in data streams to give more detailed timing information.
These and various other features as well as advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.